In many areas of the world, major problems are encountered concerning product counterfeiting, unauthorized distribution and sale of products (e.g. grey market trading, parallel trading, product diversion), as well as false liability based on product substitution.
In addition to product counterfeiting, product adulteration is another major problem. Product adulteration takes place when a product is tampered with, such as by dilution. An example of such a problem lies in the adulteration of lubricating oils, or other oil-based products, by addition of a counterfeiter's oil to a genuine product. Such adulteration is not only financially damaging to the oil manufacturer but the consequent lowering of performance which can occur can cause damage to the consumer and consequently harm the reputation of the genuine product.
It is known that various liquid products can be marked using colorants or covert systems in order to make their misuse impossible or at least traceable. Such marking can, for example, trace the identity of liquids, identify various grades, or distinguish manufacturer's brands. Various problems have on occasion accompanied the use of dyes or colorants as markers for liquid products.
In order to detect the presence of a covert marker, many of the existing markers must be extracted by a chemical process. Chemical detection normally requires extraction of the marker with an acidic or basic aqueous liquid extractant, followed by addition of a reagent to cause the extract to turn a visibly distinct colour, although in some cases, the reagent is unnecessary. While effective, this procedure has some drawbacks. For instance, it is time-consuming to perform and often does not provide a good quantitative measurement of marker concentration in field tests.
Some covert markers are organic molecules which either absorb or fluoresce in the near infrared to mark their presence in a liquid sample. U.S. Pat. No. 5,525,516 (Eastman Chemical) and German Patent DE4224301A1 (BASF) describe such markers. While the detection procedure is much simpler, some liquids naturally contain compounds that interfere with the spectrophotometric measurements, potentially compromising accurate quantitative detection.
Detection methods which employ fluorescent labels are of limited sensitivity for a variety of reasons. First, with conventional fluorophores it is difficult to differentiate between specific fluorescent signals and nonspecific background signals. Most common fluorophores are aromatic organic molecules that have broad absorption and emission spectra, with the emission maximum red-shifted 50-100 nm to a longer wavelength than the excitation (i.e., absorption) wavelength. Typically, both the absorption and emission bands are located in the UV/visible portion of the spectrum. The lifetime of the fluorescence emission is generally short, on the order of 1 to 100 ns. These general characteristics of organic dye fluorescence are also applicable to background signals, to which other naturally occurring molecules may contribute, or the sample itself (Jongkind, et al., Exp. Cell Res. 138:409, 1982; Aubin, J. E., J. Histochem. Cytochem. 27:36, 1979). Therefore, the limit of detection of specific fluorescent signal from typical fluorophores is limited by the significant background noise contributed by nonspecific fluorescence and reflected excitation light.
A second problem of organic dye fluorophores that limits sensitivity is photolytic decomposition of the dye molecule (i.e., photobleaching). Thus, even in situations where background noise is relatively low, it is often not possible to integrate a weak fluorescent signal over a long detection time, since the dye molecules decompose as a function of incident irradiation in the UV and near-UV bands.
A third problem of organic dye fluorophores is that quantitation of the emission is limited due to quenching. For example, energy that is normally released as light energy can be absorbed by intermolecular collisions with the solvent. The amount of quenching experienced by a fluorophore in a liquid sample is highly variable, and can depend on a number of factors, such as temperature, solvent, and possible energy absorbing contaminants in the solvent. As a result, unless the solvent and conditions are highly controlled, true quantitation is difficult to achieve.
When a phosphor or other luminescent material emits light, in general, it emits light according to Stokes' Law, which states that the wavelength of emitted light is always longer than the wavelength of the exciting radiation. While Stokes' Law holds for the majority of cases, it does not hold in all instances. For example, in some cases, the wavelength is the same for both the absorbed and the emitted radiation. That is, the efficiency appears to be perfect or unity. This is known as resonance radiation. Stokes' Law also does not hold when the energy emitted is greater than the energy absorbed, with the emitted light known as an anti-Stokes emission. Anti-Stokes materials typically absorb infrared or near infrared radiation in the range of about 700 to about 1500 nm, and emit light in the near infrared red or visible spectrum. The use of anti-Stokes materials in security documents (for example, European Patent EP 1241242—Bundesdruckerei), and for the authentication of polymers (Hubbard, et al., U.S. Pat. No. 6,514,617), has been described. However, the use of such materials for the identification and/or authentication of liquids has not been described.